This invention relates in general to substrate coating and in particular to a new and useful process for depositing a layer on substrates in a vacuum chamber.
The present invention concerns a process for depositing a layer on substrates in a vacuum chamber by which the chemical reactants of the substance forming the layer are introduced in gaseous form into the vacuum chamber, activated and partly ionized by an electrical gas discharge in the space in front of the substrate surface to be coated and applied to the substrate surface for the layer-forming reaction.
The deposition of metals or compounds from a gaseous phase reaction has been known for a long time and has become a widely used industrial manufacturing technique during the last decades. In the most widely used thermochemical and pyrolytic variant, gases are decomposed into layers on hot surfaces. The processes with inorganic starting materials, e.g. the metallic halides, require relatively high temperatures which does not allow their application to most of the substrate materials. Typical temperature for these are 800.degree.-1,000.degree. C., i.e. temperatures that are above the tempering temperatures of most steels and are also unsuitable for non-ferrous metals or aluminum alloys.
To circumvent this problem, less stable organometallic compounds have been used as starting materials until now and/or the reaction partners were activated by an electric gas discharge in the space in front of the surface to be covered by a layer. When such reactions are carried out in a high-frequency gas discharge, the reaction products frequently precipitate in the form of a fine powder, depending on the pressure and temperature conditions in the reaction chamber (German Pat. No. 731,318), but they can also be precipitated as coating on substrate surfaces. When the temperature is lowered, which was possible with high-frequency excitation and with the use of readily decomposable organometallic reactants, the pressure in the reaction chamber must be reduced correspondingly also to adjust a proper equilibrium between the coating of the substrate surfaces on the one hand and the powder synthesis in the gas space on the other hand.
But as recent accidents have demonstrated, the use of organometallic compounds is hardly justifyable because of their poisonous character. Consequently during the last few years the only practical alternative to the purely physical coating processes such as metallizing under vacuum or cathode evaporation, was the activation by an HF-discharge. But even this HF-process for the precipitation of layers by a chemical reaction between a gas introduced into the reaction chamber and the substrate to be coated or among several introduced gaseous reactants, in which case the reaction product precipitates as deposit on the substrate surface, still has considerable disadvantages. Among these are especially the high costs of efficient generators that shielded, as required by law. The introduction of high frequency into the reaction chamber without great losses is also difficult, particularly when the parts to be covered are held by rotating carriers during the coating. Another difficulty is the requirement that, for a uniform coating, the plasma density be as homogenous as possible in the space in front of the surface to be coated, which frequently cannot be achieved with parts of complex shapes, e.g. tools, however. Differences in the thickness and the quality of the layer at various points of the same piece of work are the consequence.
This was the reason for attempts to perform such coating processes in direct-current discharges of the diode type, but these failed because of the high accelerating voltags for the (electrically charged) radicals occurring during this process, which probably resulted in undesirable secondary reactions.
For the so-called ion plating, metals are vaporized by heating and simultaneously ionized to a small degree in a plasma, by which the produced positive ions are accelerated in the direction of the substrates laid on a negative potential. The result are coatings with better adhesion to the substrates. The depositing of chemicals is also possible by this method, provided that an active gas is introduced simultaneously into the vaporizing chamber. For example, nitride layers can be deposited by vaporizing titanium in a nitrogen atmosphere.
But this process could be used only for the production of those compounds that could be obtained by the reaction of a metal vaporized at a high temperature with the active gas atmosphere. The high temperature of the vaporized metal resulted in high substrate temperatures due to heat radiation, necessitating costly cooling and making the coating of some substrates with compounds from metals with high melting points impossible by this method. The generating and handling of the metallic melt in the active atmosphere is also costly and difficult.
A summarizing description of the process for the preparation of metallic protective coatings by gas phase reactions (chemical vapor deposition-abbreviation CVD used in the following text) is found, e.g., in Int. Metals Reviews 1978, No. 1, p. 19. The preparation of non-metallic coatings by plasma-supported deposition from the vapor phase is described in a report by K. R. Linger in AERE, Harwell, Didcot, Oxon, OX 11 ORA.